On a switching network at a data link layer, when a switch transmits packets, if a loop exists on the network, some packets continuously multiply and endlessly cycle on a loop network, and therefore packet processing capability of the switch is reduced because a same packet is repeatedly received.
To prevent occurrence of the foregoing problem, the Spanning Tree Protocol (STP) emerges, where the protocol is established according to an Institute of Electrical and Electronics Engineers (IEEE) standard, and is used to collect and eliminate, on a local area network, physical loops at a data layer. Switches that run the protocol discover a loop on a network by exchanging information with each other, and selectively block some ports. Finally, a loop network structure is built into a loop-free tree network, thereby preventing occurrence of a broadcast storm. Generally, a protocol packet used by the STP is a bridge protocol data unit (BPDU). The BPDU includes sufficient information to ensure that a device completes a calculation process of a spanning tree, and the STP determines a topology structure of a network by transmitting the BPDU between devices. FIG. 1 is a schematic diagram of a packet format of a BPDU in the prior art. As shown in FIG. 1, an identifier (ID) of a specified port includes a priority and a port number of the specified port, and occupies a total of 2 bytes, where the priority of the specified port occupies 4 bits, and the port number occupies 12 bits. That is, the switch may use a maximum of 4095 port numbers.
However, in a super virtual fabric (super virtual switching network), multiple switches need to be virtualized into one logical device, to provide unitary configuration and management outwards. On the virtual switching network, a quantity of ports of the logical device may exceed 4095. However, currently port numbers of a switch occupy only 12 bits, and cannot identify all ports.